Defining plant growth promoting rhizobacteria molecular and biochemical networks in beneficial plant-microbe interactions

Background Our knowledge of plant beneficial bacteria in the rhizosphere is rapidly expanding due to intense interest in utilizing these types of microbes in agriculture. Laboratory and field studies consistently document the growth, health and protective benefits conferred to plants by applying plant growth promoting rhizobacteria (PGPR). PGPR exert their influence on other species, including plants, in the rhizosphere by producing a wide array of extracellular molecules for communication and defense. Scope The types of PGPR molecular products are characteristically diverse, and the mechanisms by which they are acting on the plant are only beginning to be understood. While plants may contribute to shape their microbiome, it is these bacterial products which induce beneficial responses in plants. PGPR extracellular products can directly stimulate plant genetic and molecular pathways, leading to increases in plant growth and induction of plant resistance and tolerance. This review will discuss known PGPR-derived molecules, and how these products are implicated in inducing plant beneficial outcomes through complex plant response mechanisms. Conclusions In order to move PGPR research to the next level, it will be important to describe and document the genetic and molecular mechanisms employed in these interactions. In this way, we will be able to restructure and harness these mechanisms in a way that allows for broad-based applications in agriculture. A greater depth of understanding of how these PGPR molecules are acting on the plant will allow more effective development of rhizobacterial applications in the field.